Marla walks 23 blocks. The next two days she walks 17 and 12 blocks. How many blocks Marla walk in all?

The correct answer is: 52

Marla walks 23 blocks. The next two days she walks 17 and 12 blocks. 
Total walk = 23 + 17 + 12
To find partial sums add the digits in tens place and ones place seperately.
Partial sum of 23 + 17 + 12
Step 1 : Add digits in tens and ones place seperately:
20                                  03
+  10                               + 07
+  10                 and        + 02

_____                             ______
40                                       12
Thus, the sum of 23 + 17 + 12are 40 + 12 = 52.
So, Marla walks 52 blocks in three days.